Early in the history of terrestrial planets, the fractional crystallization of a magma ocean can lead to a mantle stratification characterized by a progressive enrichment in heavy elements from the core-mantle boundary to the surface. Such a configuration is gravitationally unstable; it causes mantle overturn and the formation of a stable chemical layering. Using simulations of thermochemical convection, we analyzed the consequences of overturn and subsequent layering on mantle dynamics assuming Mars' scaling parameters. We found that the time needed to achieve chemical homogenization via convective mixing scales exponentially with the buoyancy ratio , which measures the relative importance of chemical to thermal buoyancy. In addition, when using a strongly temperature-dependent viscosity, the formation of a stagnant lid prevents the uppermost crystallized layers from sinking into the mantle. In order to obtain their subduction, a yielding mechanism must be invoked. In the context of Mars' evolution, our results suggest that complete chemical mixing is unlikely to take place within time scales comparable with the planet's age. Magma ocean freezing could thus be responsible for the long-term preservation of compositional heterogeneities as required by meteoritic evidence. The inferred lack of a high density lid is difficult to reconcile with a stagnant-lid regime operating throughout Mars' history. An episode of surface mobilization induced by compositional overturn can resolve this difficulty provided that is large enough. Too large buoyancy ratios, however, tend to suppress convective heat transport, rendering it problematic to explain the late volcanic history of Mars.
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